The present invention relates in general to diesel engine filtration systems and in particular to a coalescing filter to remove oil aerosol from a blowby gas (exhaust) stream. More specifically, the present invention relates to a coalescing filter which is subjected to rotation in order to expel the coalesced liquid from the filter and thereby keep any flow restriction within the filter comparatively low.
In order to achieve high separation efficiency for oil aerosol in the 0.1-1.0 micron size range, it is necessary to use a relatively "tight" coalescing medium which is constructed from very fine fibers (melt-blown or glass). A consequence of fine fibers is the corresponding fine pore size distribution. The presence of fine pores in a coalescing filter can result in the pores becoming "clogged" with the liquid being separated, due to the surface tension and the corresponding "bridging" effect. This relatively high surface tension causes a correspondingly high restriction since it takes a large pressure to overcome the surface tension across a small wetted pore. It is known that the pressure required to "blow out" a pore is inversely proportional to the pore diameter. This behavior has been clearly verified by testing with various grades of media. What has been learned is that the pressure required to break through the film of a wetted pore is several times higher than the "dry" restriction at design face velocity. The lowest reported difference in wet flow restriction compared to dry flow restriction was a three-fold increase in flow restriction for the wetted condition.
Since engine crankcase pressure must be kept very near atmospheric pressure, approximately 5 inches of water, it is difficult to design a high-efficiency coalescer without resorting to a fairly elaborate arrangement of pressure control valves, vacuum assist devices, and similar mechanisms. For this reason, a means of keeping the coalescer element dry and operating at a low restriction would be a useful improvement to current filter designs. The present invention provides a solution to this identified need in a novel and unobvious manner.
The present invention includes a structural arrangement for use in cooperation with a coalescing filter for mounting the filter on a rotating component of the corresponding engine such as on a gear within a gear housing. The arrangement is configured such that the blowby gas stream must pass through the spinning coalescing element prior to exhausting to the atmosphere or to a location upstream of an air filter. By exhausting to the atmosphere or upstream of the air filter, the need for pressure control valving is eliminated. The actual filter element used to provide the improvement of the present invention may assume a pleated cylinder, disk, cone, cylinder, wafer, or tube shape, to mention only some of the possibilities.
The centrifugal force which is imparted on the coalescing filter element due to the rate of rotation causes the liquid to rapidly drain from the coalescing medium, similar to the spin-dry cycle of a washing machine. What has been found by testing is that increasing the "g" force dramatically accelerates the oil drainage, especially from tight ultra low penetration air (ULPA) media and HEPA media. An ULPA media has a 99.999 percent efficiency at 0.3 microns while the HEPA media has a 99.97 percent efficiency at 0.3 microns. At 1000 g's, the filter elements returned to their near dry weight in approximately 2 hours. In the static test at 1 g, very little drainage occurred, even in over 100 hours. Specifically for the HEPA media, as little as 100 g's resulted in a marked increase in oil drainage rate. This testing has validated the potential for the present invention to provide significant improvements in the design and utilization of coalescing filters.